1. Field of the Invention
This invention relates to lithography and more specifically to electron beam lithography for semiconductor device fabrication.
2. Description of Related Art
There are two general categories in the field of lithography. The first is photolithography (light lithography) which images patterns on a substrate, typically a semiconductor wafer, using a mask which is a pattern through which a beam of light is passed and imaged onto the surface of the substrate. The surface of the substrate carries a layer of photosensitive resist which is thereby exposed by the mask pattern. Later steps of developing the photoresist and etching the substrate are performed to form a pattern replicating the image of the mask on the wafer.
A second category of lithography is electron beam (or charged particle beam) lithography in which a beam of e.g. electrons from an electron source is directed onto a substrate. The electrons expose a resist layer (in this case an electron sensitive resist) on the substrate surface. Electron beam lithography uses what are called "electron lenses" to focus the electron beam. These are not optical (light) lenses but are either electro-static or magnetic. Typically electron beam lithography is used for making masks; however it can also be used for direct exposure of semiconductor wafers. The systems used in photolithography or electron beam lithography are well known and include a source of light or electrons, optical or electron beam lenses, and stages for supporting the substrate and the mask (reticle).
Typically electron beam lithography does not use a pattern (mask) but instead is "direct write" in which the beam is scanned and turned on and off (blanked) to determine the patterns imaged on the substrate. It is also known to use electron beams in conjunction with masks. The chief disadvantage of electron beam direct write lithography is its relatively slow exposure rate, making it generally uneconomic for semiconductor wafer fabrication.
As is well known, the primary goal in lithography in the semiconductor field is to define smaller feature sizes, where feature size is usually the minimum width of a portion of a transistor or interconnection. Generally photolithography and electron beam lithography have followed different evolutionary steps. Photolithography has achieved its present dominant position in semiconductor device fabrication by concentrating on mask techniques using a mask (reticle) which defines the actual image. These techniques utilize a highly efficient parallel projection scheme whereby a single reticle is used repeatedly to project the identical image onto different portions of the semiconductor wafer.
In contrast, typical applications of high resolution electron beam lithography are limited to mask-making and to limited manufacturing of specialized (low production) integrated circuits due to the inherent low throughput in direct write lithography and high equipment cost. However, since the general trend in semiconductor fabrication is to reduce minimum feature size progressively, it is expected that a typical minimum feature size will be less than 100 nanometers (nm) in about ten years and at that time optical lithography may become too expensive and not offer sufficient resolving power due to the relatively large wavelength of light.
At the same time, current electron beam technology is not regarded as economic even in the long term for mass production of semiconductor devices.
An improved combined light and electron lithography process and apparatus which takes advantage of the high throughput of photolithography and the high spatial resolution of electron beam lithography, is disclosed in parent application, U.S. Ser. No. 09/045,728, filed on Mar. 20, 1998, entitled "Tandem Optical Scanner/Stepper And Photoemission Converter For Electron Beam Lithography". The parent application discloses a system for carrying out the combined method by combining two subsystems, the first of which is a conventional photolithography tool, for instance a stepper or scanner, and the second of which is a demagnifying electron beam column. These two subsystems are coupled by a photoemission cathode.
The photo and electron beam subsystems are arranged serially. The photolithography subsystem transfers one to one or a demagnified image (demagnified for instance four to five times) of the conventional mask (reticle) onto the photoemission cathode, which couples the photo subsystem to the electron beam subsystem. The photoemission cathode converts the incident light (photons) into an electron beam emission pattern and the electron optics project a demagnified electron image of the mask onto the wafer surface.
The photon subsystem is based for instance on a conventional stepper or scanner of the type now commercially available, while the electron beam subsystem includes the photoemission cathode, extraction electrode and demagnifying lenses, each of which are essentially conventional. When a scanner is used in the photon subsystem, the wafer is written on the fly, i.e. both the mask (reticle) and wafer move at constant velocities in proportion to total demagnification. In the other case when a stepper is used as the photon subsystem, the wafer is written when both the mask and wafer stop. The exposure begins after the mask and wafer are moved in the appropriate position.
A unique feature of the parent application composite system is that the optical lenses of the photolithography subsystem can be used to compensate for distortion aberrations in the electron beam lens (or visa versa). Applications of the system and method in accordance with the parent application include electron beam lithography tools for electron beam direct writing of wafers and for mask making with high throughput by combining photolithography and high resolution electron beam lithography for exposure.